Analyze the parameter setting techniques for CNC turning threads

When CNC turning threads, parameter Settings directly affect the processing quality and efficiency of the threads. The following analyzes parameter setting techniques from four aspects: basic thread parameters, cutting parameters, tool parameters, and programming parameters:

Basic parameters of threads

Pitch (P) : Pitch is the axial distance between two corresponding points on the centerline of adjacent threads and is one of the important parameters of a thread. When setting the pitch, it is necessary to input strictly in accordance with the requirements of the design drawing. For example, if the drawing requires a pitch of 1.5mm, this value must be accurately input in the numerical control system. If the pitch is set incorrectly, the machined thread will not fit properly with other parts.

Thread profile Angle (α) : The thread profile Angle is the Angle between the sides of two adjacent threads on the thread profile. Common examples include the 60° thread profile Angle of ordinary threads, etc. The size of the tooth profile Angle determines the shape and fit performance of the thread. Before processing, it is necessary to confirm the thread profile Angle, select the appropriate cutting tool and adjust the parameters of the numerical control system. For example, when processing common threads, it is necessary to ensure that the tip Angle of the tool matches the thread profile Angle. At the same time, during programming, it is also necessary to ensure that the movement trajectory of the tool meets the requirements of the thread profile Angle.

The maximum diameter (D) and minimum diameter (d₁) of the thread: The maximum diameter of the thread refers to the diameter of the imaginary cylinder tangent to the top of the external thread or the bottom of the internal thread, while the minimum diameter refers to the diameter of the imaginary cylinder tangent to the bottom of the external thread or the top of the internal thread. When programming, the starting position of the tool and the machining depth should be determined based on the large and small diameters of the thread. For instance, when processing external threads, the starting position of the cutting tool should be slightly larger than the maximum diameter of the thread, and the processing depth should be determined based on the difference between the minimum and maximum diameters of the thread.

Cutting parameters

Spindle speed (n) : The selection of spindle speed needs to comprehensively consider factors such as thread pitch, tool material, and workpiece material. Generally speaking, the spindle speed should not be too high; otherwise, the excessive cutting speed will lead to accelerated tool wear and even the formation of built-up edge, affecting the surface quality of the thread. At the same time, the spindle speed should not be too low; otherwise, the processing efficiency will be reduced. For example, when processing common threads of steel workpieces, the spindle speed is usually controlled at about several hundred revolutions per minute. The specific rotational speed can be estimated based on the empirical formula n = 1200/P - k (where P is the pitch and k is the correction factor, determined according to the tool and workpiece materials).

Feed rate (F) : When CNC turning threads, the feed rate is closely related to the spindle speed and pitch. Generally, the feed rate F is equal to the pitch P multiplied by the spindle speed n, that is, F = P×n. This is because in thread processing, for each revolution of the tool, it needs to move a distance of one pitch along the axial direction to ensure the pitch accuracy of the thread. For example, when the pitch is 1.5mm and the spindle speed is 300r/min, the feed rate F = 1.5×300 = 450mm/min.

Cutting depth (ap) : The cutting depth should be determined based on factors such as the precision requirements of the thread and the hardness of the workpiece material. Generally speaking, a larger cutting depth can be adopted during rough machining to quickly remove most of the allowance. When performing fine machining, a smaller cutting depth should be adopted to ensure the dimensional accuracy and surface quality of the thread. For instance, during rough machining, the cutting depth can be taken as 0.5-1mm, and during finish machining, it can be taken as 0.1-0.2mm.

Tool parameters

The geometry of the cutting tool: The geometry of the thread turning tool has a significant impact on the processing quality of the thread. The tip Angle of the cutting tool should be consistent with the thread profile Angle, the cutting edge should be sharp, and the rake Angle and relief Angle should be appropriate. For instance, when processing common threads, the tip Angle of the tool is usually 60°, the rake Angle is generally between 5° and 15°, and the relief Angle is between 6° and 8°. If the tip Angle of the knife is inaccurate, it will lead to incorrect thread profile. If the cutting edge is not sharp or the rake Angle and relief Angle are inappropriate, it will increase the cutting force and affect the surface quality of the thread.

Tool installation: When installing the tool, ensure that the tool tip is at the same height as the workpiece axis; otherwise, it will affect the machining accuracy of the thread. If the tool tip is higher than the axis of the workpiece, the actual maximum diameter of the thread will increase. If the tip of the tool is lower than the axis of the workpiece, the actual maximum diameter of the thread will be reduced. At the same time, the installation of the cutting tools should be firm to avoid vibration during the processing.

Programming parameters

The starting and ending coordinates of the thread: When programming, it is necessary to accurately determine the starting and ending coordinates of the thread. The starting point of the thread should be determined based on the clamping position of the workpiece and the initial position of the tool, and the ending point should be determined based on the length of the thread. For example, if the thread length is 20mm and the starting position of the tool is 5mm away from the end face of the workpiece, then the starting coordinate of the thread can be set as Z-5 and the ending coordinate can be set as Z-25.

Thread cycle instructions: Different numerical control systems have different thread cycle instructions, such as G92, G76, etc. The appropriate thread cycle instruction should be selected according to the numerical control system used, and the parameters in the instruction should be set correctly. For example, when using the G92 command, parameters such as the pitch of the thread, feed rate, and cutting depth need to be set; When using the G76 command, parameters such as the thread profile Angle and the finish allowance of the thread also need to be set.

Multiple tool passes: To ensure the machining quality of threads and the tool life, multiple tool passes are usually required. When programming, it is necessary to reasonably arrange the cutting depth and feed rate for each tool pass. For example, for the first tool pass, a larger cutting depth and a lower feed rate can be adopted to quickly remove most of the allowance. The subsequent tool passage gradually reduces the cutting depth and increases the feed rate for finish machining.